Ask them to write down their goals for the pond - all uses and purposes. Ask for the info from the soil tests and tell them you need it as it limits (sets the parameters of) what can be done biologically. Ask if they will be aerating as that determines the ability of the deep water to hold O2 for the fish and thus set the ponds carrying capacity. Check on ponds in the area to see the fertility . With that you can do a budget but tell them availability and cost of fish varies a lot based on time , type and size of fish needed. Find out what services you will be providing , if any, other than stocking.